By Park Boram SEOUL, March 17 (Yonhap) -- Three years after North Korea's deadly attack on a South Korean Navy ship in the Yellow Sea that killed 46 sailors, the communist country has ratchet up tensions and is posing a much greater security threat to Seoul along the sea demarcation line that separates the two Koreas.

March 26 marks the third anniversary of the sinking of the corvette Cheonan southwest of Baengnyeong Island. An official investigative team set up by the government said a torpedo, probably launched by a North Korean mini submarine, tore the 1,200-ton ship in two in the night attack.

The provocation triggered punitive measures that effectively cut inter-Korean exchanges and economic cooperation, and forced South Korea to build up its defensive capabilities along the Northern Limit Line (NLL) that acts as the de facto border at sea.

Despite steps taken and Seoul's repeated warnings that it will strike back hard if provoked in the future, the North is now showing signs it may be ready to provoke the South once again. According to official reports, the western sea border region has borne the brunt of military provocations from the North. Of the 535 military provocations carried out since 1990, 77 percent occurred in the region. It is the site of three bloody naval battles since 1999 that resulted in the loss of life for both sides.

"The most grave concern is possible attacks carried out by North Korean military forces," a South Korean military official said, adding that preparations have been made to counter guerrilla (attacks) to occupy border islands in the Yellow Sea or any new types of provocative acts aimed at the South's armed forces.

According to South Korean military officials, the North has recently increased operations of its submarine fleets, is doing more to train its coastal artillery units and may have even beefed up secret infiltration missions targeting the South.

Since last year, the North Korean military has intensified amphibious training involving ground, naval and air force elements, and focused on tactics to attack the group of South Korean border islands lying along the NLL.

Besides Baengnyeong, which has a population of about 5,000, Daecheong, Socheong, Yeonpyeong and Udo are the so-called Seohae Islands that could be the target of a North Korean attack.

South Korea has also detected far more frequent activities of the North Korean semi-submersible Taedong-B boats in the northern region of the NLL in the Yellow Sea since late last year, military officials said. The semi-submersible boats are equipped with two torpedo launch tubes and are difficult for radar to detect because of their design. Such boats can attack South Korea surface combatants in coastal waters.

The number of infiltration exercises using submarines has also increased by three-to-four fold, according to the military.

A total of 13 North Korean submarines armed with torpedoes, along with 360 smaller fighting vessels, are known to be stationed along the NLL to counter the South Korean Navy.

Officials also noted that as many as 1,000 North Korean artillery pieces may be targeting the Seohae Islands that can do considerable damage.

"Threats of provocations targeting the five border islands have been steadily intensified," a Navy official, who declined to be identified, said. The North Korean threats are growing more in intensity, he claimed.

In response to the escalating risk of North Korean provocations following the Cheonan ship sinking, South Korea upgraded its defense capacities along the Yellow Sea border. The country's Navy has patrol ships equipped with torpedo counter measures that are far more advanced than the equipment the Cheonan had when it was struck by a torpedo.

Seoul, moreover, dispatched additional aircraft that can deal with anti-submarine and surface threats.

The South Korean military is also moving to secure Spike missile system from Israel to better eliminate North Korea's coastal artillery, a Navy official said.

The preparations are viewed as necessary as the inter-Korean military tensions have risen to new highs as the North has repeatedly threatened to launch a nuclear attack on the U.S. and the South after the recent adoption of punitive United Nations resolutions and the on-going joint military drills between the two allies.

The U.N. Security Council adopted two resolutions this year to penalize Pyongyang for launching a long-range rocket in December and an underground nuclear test on Feb. 12. Seoul and Washington are engaged in the Key Resolve Foal Eagle exercise that Pyongyang claims is a dress rehearsal to invade the North with nuclear weapons.

Highlighting the warlike rhetoric against the South, North Korea's media reported last week that its leader Kim Jong-un guided artillery exercises that trained artillery units to target two South Korean islands in the Yellow Sea. Kim who is thought to be in his late 20s or early 30s inherited power after the sudden death of his father in late 2011. Many North Korean watchers in Seoul think he may be fueling tensions to help solidify his power base that is not as strong as his father's was.